Device and method for exhaust gas after-treatment

ABSTRACT

A device and a method for treating the exhaust gas of an internal combustion engine are proposed, comprising an ozone reactor for supplying ozone to the exhaust gas at a delivery location ( 54 ), an oxidation reactor ( 20 ) for the at least partial oxidation of nitrogen oxides and/or hydrocarbons being positioned upstream from the delivery location ( 54 ), the oxidation reactor being active independently of the operating state of the internal combustion engine, especially also at the exhaust-gas temperatures prevailing at the start or during the warm-up phase of the internal combustion engine.

BACKGROUND INFORMATION

[0001] The present invention is directed to a device or a method fortreating the exhaust gas of an internal combustion engine according tothe species defined in the independent claims. A device for exhaust-gastreatment is already known from DE 199 04 068 in which ozone produced byan ozone reactor is introduced into the exhaust system of an internalcombustion engine. However, at low temperatures a combustion of dieselsoot is not ensured since the oxidation catalyst upstream from theparticle filter does not oxidize nitrogen oxides at low temperatures,especially below an exhaust-gas temperature of 250 degrees Celsius. As aresult, the ozone delivered by the ozone reactor is reacted away by thenitrogen oxides, so that it is unable to have any effect as an oxidantin the particle filter.

SUMMARY OF THE INVENTION

[0002] In contrast, the device and the method according to the presentinvention have the advantage over the related art that a soot combustionis ensured even at low exhaust-gas temperatures, especially atexhaust-gas temperatures below 250 degrees Celsius, that is, undercold-start conditions. Moreover, providing two separate branches for theoxidation of nitrogen oxides and for the supply of ozone ensures anenergetically advantageous procedure, since the energy quantitiesrequired for oxidizing the nitrogen oxides are less than those neededfor ozone generation and, energy-wise, each path is able to be optimizedon its own as a result of the separation. Especially advantageous is theprovision of a downstream particle filter as an exhaust-gas treatmentunit whose regeneration is ensured in all operating states of the engineby the combination of an ozone reactor with a plasma reactor as proposedby the present invention.

[0003] Further advantages are derived from the additional features namedin the dependent claims and the description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Exemplary embodiments of the present invention are shown in thedrawing and explained in detail in the following description. The singleFIG. 1 illustrates a device and a method for exhaust-gas treatment whichuses two separate branches for the oxidation of nitride oxides and forthe generation of ozone, respectively.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0005]FIG. 1 shows an ozone reactor 30 which is connected on the intakeside to an air supply 4 for the infeeding of air 49, and at the outputside to an ozone line 6 for the supply of ozone-enriched air 50 to anexhaust line 5 at a delivery location 54. Exhaust-gas line 5 is able tobe connected to an internal combustion engine (not shown), especially adiesel combustion engine, to carry away exhaust gas 51. A plasma reactor20 is situated upstream from delivery location 54 in exhaust-gas line 5.Exhaust gas 52 processed in plasma reactor 20 mixes with ozone-enrichedair 50 at delivery location 54 and flows through exhaust-gas treatmentunit 10 which is configured as a particle filter. Downstream from theexhaust-gas treatment unit, purified exhaust gas 53 leaves the unit.Both ozone reactor 30 and plasma reactor 20 are reactors for producing anon-thermal plasma, i.e., a gas having relatively cold ions and hot freeelectrons, that is, high-speed electrons, so that oxygen radicals, inparticular, are produced and oxygen molecules and nitrogen monoxides areable to be oxidized into ozone and nitrogen dioxide, respectively. Theplasma is produced in a generally known manner in a silent electricaldischarge (Corona discharge or dielectric barrier discharge), the gas tobe treated flowing between two electrodes, and between which anelectrical high-voltage alternating field may be applied. In thisconnection, a dielectric is applied to at least one electrode and/or theelectrode geometry is selected in such a way that field inhomogeneitiesresult which prevent a continuous and sustained spark-discharge, so thatthe heavy particles, i.e., the ions, are unable to heat up and only thedesired silent discharges occur to accelerate the electrons.

[0006] Due to the low thermal stability of ozone, this reactivesubstance is produced by oxidation of the air oxygen in a separate airauxiliary branch (4, 30, 6) whose temperature approximately correspondsto the external temperature of the motor vehicle housing the internalcombustion engine. In order to avoid ozone losses due to the oxidationof nitrogen oxides and hydrocarbons contained in diesel exhaust gas 51,plasma reactor 20 is provided between the internal combustion engine anddelivery location 54. It oxidizes, the hydrocarbons contained in theexhaust gas at least partially; nitrogen monoxide is oxidized intonitrogen dioxide as well. These oxidation processes both in ozonereactor 30 and in plasma reactor 20 are brought about by the silentelectrical discharges which extend along individual filament-likeregions between the electrodes in intervals of approximately 50nanoseconds. In the process, UV light is produced as well, whichadditionally aids in the oxidation processes. The electrical energy tobe fed into the plasma reactor is less than the energy required forozone reactor 30, so that, in particular, smaller field amplitudes and,thus, voltage amplitudes, suffice in plasma reactor 20 to oxidize thenitrogen monoxides generated per unit of time. By converting thenitrogen oxides into nitrogen dioxide prior to the addition of ozone,the produced ozone is fully retained in the exhaust branch up to thediesel particle filter. This makes it possible to oxidize soot in thediesel particle filter into carbon dioxide with the aid of ozone,especially in continuous operating mode.

[0007] In alternative embodiments, the ozone and/or the plasma reactormay also include electrode systems in which both electrodes are coveredby a dielectric (two-sided instead of one-sided dielectric barrierelectrical discharge). Alternatively, the barrier to the electricaldischarge for obtaining silent discharges may also be achieved byappropriate electrode geometries that cause field inhomogeneities,thereby preventing continuous spark discharges. This is assured, forinstance, by a cylindrical electrode arrangement in which one of theelectrodes is concentrically configured as a rod in a cylinder throughwhich gas flows and which constitutes the second electrode.Alternatively, instead of an electrical a.c. voltage in the kHz-range, apulsed direct voltage may be applied as well. In this case, however, apulse-pause ratio has to be provided whose minimum value is defined bythe flow velocity of the exhaust gas and whose maximum value ispredefined by the duration required for evenly dispersing again theelectrons present on an employed dielectric. In an additionalalternative embodiments a system may be provided as an ozone and/or as aplasma reactor which has an UV lamp as the essential element forinitiating oxidation processes.

What is claimed is:
 1. A device for treating the exhaust gas of aninternal combustion engine, comprising an ozone reactor for deliveringozone to the exhaust gas at a delivery location, wherein an oxidationreactor (20) is positioned upstream from the delivery location (54) forthe at least partial oxidation of nitrogen oxides and/or hydrocarbons,the oxidation reactor being active independently of the operating stateof the internal combustion engine, especially also at exhaust-gastemperatures prevailing at a start or during the warm-up phase of theinternal combustion engine.
 2. The device as recited in claim 1, whereinthe oxidation reactor is a plasma reactor (20).
 3. The device as recitedin claim 2, wherein the plasma reactor is a reactor for producing anon-thermal plasma.
 4. The device as recited in claim 3, wherein thenon-thermal plasma is produced by an electrical discharge and/or a UVlamp.
 5. The device as recited in claim 4, wherein the electricaldischarge is a silent electrical discharge.
 6. The device as recited inclaim 5, wherein the silent electrical discharge is a dielectricalbarrier electrical discharge.
 7. The device as recited in one of thepreceding claims, wherein the ozone reactor (30) is formed by anadditional plasma reactor.
 8. The device as recited in claim 7, whereinthe additional plasma reactor is a reactor for producing a non-thermalplasma.
 9. The device as recited in one of the preceding claims, whereinan exhaust-gas treatment unit is, situated downstream from the deliverylocation in the direction of the exhaust-gas flow.
 10. The device asrecited in claim 9, wherein the exhaust-gas treatment unit is a particlefilter (10).
 11. The device as recited in claim 10, wherein the particlefilter is catalytically coated.
 12. A method for treating the exhaustgas of an internal combustion engine in which ozone is added to theexhaust gas to be purified, wherein, prior to the addition of ozone,nitrogen oxides and/or unburned hydrocarbons contained in the exhaustgas are at least partially-oxidized post engine, the oxidation beingcarried independently of the operating state of the internal combustionengine, especially also at the exhaust-gas temperatures prevailing at astart or during the warm-up phase of the internal combustion engine.